1. Field of the Invention
The present invention relates to a method and apparatus for use in removing liquid from paper in the press section of a paper making machine.
2. Description of the Prior Art
In the manufacture of paper, after the paper sheet has been formed, it is subjected to mechanical compaction in order to express water therefrom. Water left in the sheet following final mechanical compaction must be removed by evaporative drying. The energy cost associated with the aforementioned evaporative drying is very high, in fact many times higher than is required for mechanical compaction.
The known apparatus for carrying out the aforementioned process in one instance comprises a press-plate adapted to squeeze a stationary formed sheet of paper and, in a further instance, a pair of press rollers arranged to squeeze a moving sheet of paper, i.e., a paper web. In both instances, a combination of felt and vacuum box are used to "receive" the expressed water.
Thus, in known wet pressing operations, as exemplified by the aforementioned static and dynamic systems, there are two essential elements present, i.e., means t apply a mechanical compacting load and means to "receive" water expressed from the wet sheet being compacted.
Either of the aforementioned systems, examined when the maximum compacting pressure has been applied to the wet sheet, will reveal the following picture.
The wet sheet has been compacted (reduced in thickness) so that, first, water has displaced air in all remaining spaces thus eliminating the air, and, second, the pore volume is reduced and thus only water remains in such pores. Under these conditions, the water content of the sheet will be that which is required to saturate (fill all available pores) the compressed sheet of paper; and this quantity of water will depend on the compacting pressure, the extent to which the paper sheet can be compressed (its wet compressibility properties), and in the dynamic case, the extent to which water has had the time to be forced out of the sheet.
Independent of all these details, however, it is possible to assign for any wet pressing operation the minimum residual water content the operation is capable of producing. It is the water content required to saturate the sheet when it has been compacted to the extent imposed by the wet pressing system.
This minimum theoretical water content is never achieved in current practice because in the process of removing the applied pressure, the paper sheet or web tends to suck water back from the felt or other reservoir as it expands. This phenomenon is well recognized, and steps have been taken to minimize this "rewetting". To the extent that these steps are successful, the wet pressing operation approaches the above indicated theoretical limit.
It is, therefore, an important aim of the present invention to provide an improved, more efficient apparatus to those presently available, being ones which will overcome the aforementioned disadvantages. Accordingly, it is a further important aim of the present invention to provide an apparatus which will enhance the water removal capability of a wet pressing operation.